Incandescent lamps have long been used for illumination and signalling in a variety of applications, including on vehicles. Most applications require that the incandescent lamp be turned on and off in response to a manual actuation or a control circuit such at a signal flasher which automatically and periodically turns the lamp on and off.
Switching devices, which are used to control the application of electrical power to an incandescent lamp or other load, require overload protection so that the switching device will not be damaged or destroyed in the event of an overload, such as a short circuit or other reduced load impedance.
For conventional lamp flashers or other switches, the overload problem is solved by using a series connected fuse. Fuses are practical and solve the problem for mechanical switching devices, including bimetallic flasher switches, and for transistor flashers which use bipolar transistors. Although a fuse requires a substantial time delay until it opens, mechanical switches and bipolar transistors are not damaged within that time delay interval because they can dissipate the excess heat caused by the overload during that time interval.
However, such traditional switching devices are inefficient because they consume significant electric power. This is particularly undesirable in service vehicles which have auxiliary, electrically powered equipment which is powered from a storage battery and vehicle generators which regenerate the stored energy.
The semiconductor device manufacturing art has recently developed technology which has made power field effect transistors available which can operate at the operating currents of incandescent lamps and yet have the extremely high power efficiency which is characteristic of field effect transistors.
There is, therefore, a need for a load driver or switching circuit which can take advantage of the energy efficiency of the power FET and yet will provide for overload, short circuit, and reverse voltage protection.
For example, a lamp or other load driver should be overload proof so that, if a lower impedance load is connected to the driver than the driver is designed to handle, the device will cease operating without any damage. The load should be protected and the switching device should be protected in the event the load is short circuited and an extremely high current is demanded by the short. If the load is short circuited, the driver would desirably turn off before any damage can occur. Finally, it is desirable that the device, particularly if used as a two terminal flasher, be reverse voltage protected in the event it is incorrectly installed.
These advantages and features are attained in embodiments of the invention. In addition, the invention also provides a "soft" turn on characteristic which gradually turns an incandescent lamp on in a manner which is recommended by incandescent lamp manufacturers.